The present invention relates to a single electron device; and, more particular, to a single electron transistor including weak links with bottleneck figure and etching damage, in the place of the conventional tunnel junctions of a single electron transistor, made from an ultra-thin metal film based on simple processes combined of lithography and etching processes. The single electron transistor is promising to embody integrated single electron circuits. The present invention also relates to a method for fabricating the same.
A single electron device is an ultimate scheme of electronic device in the purpose of controlling current with one electron. Concept about a single electron transistor similar to the conventional field effect transistor (FET) had already been proposed and there have been proceeded the researches about the devices in order to embody ultra large scale integrated memories or ultra low power digital circuits. There have also been proceeded the researches about the other various new functional devices or circuits using the same principle.
An example of the single electron device is a single electron transistor similar to the conventional FET, which will be described referring to FIG. 1 schematically depicting it.
A very small electron island 120 is coupled with two nodes 110 and 130 through two tunnel junctions 115 and 125, respectively, and coupled with an input node 140 through a capacitor 135. The tunnel junctions between the electron island and the respective two nodes 110 and 130 are characterized by the resistances and capacitances of (R1, C1) and (R2, C2), respectively. A constant voltage, V0, is biased at the node 110, and a control voltage Vg is input at the node 140 of the capacitor 135 to control the characteristics of the electron island.
Such a structure is very similar to the conventional MOSFET. The two nodes 110 and 130 correspond to the source and drain respectively, and the input node 140 also corresponds to the gate.
FIG. 2 is a graph showing the characteristics of the single electron transistor as described above. The drawing shows the relation of the control voltage Vg and the current I through the electron island via the tunnel junctions when a voltage Vo is biased.
When the constant voltage Vo is input at the node 110 and the voltage Vg is input at the input node, namely gate 140, the current I is a dependent function of the voltage Vg with peak patterns having a period of e/Cg. Here, the peak corresponding to MAX is a conducting state released of the Coulomb blockade, and the part of MIN is an insulating state derived from the Coulomb blockade. The drawing shows that the current is a period function of the voltage Vg with a period of e/Cg and that the charge amount induced by the capacitor 135 can be detected with the sensitivity as little as an elementary charge e. This means that the source-drain current is modified by the induced charge amount of an elementary charge. Accordingly, this is called as a single electron transistor.
The characteristics of the tunnel junctions are given with the resistors and capacitances of (R1, C1) and (R2, C2). Assumed that the capacitance of the capacitor 135 is given as Cg, the conditions in which the phenomena shown in FIG. 2, namely, single electron tunnel phenomena occurs, are as follows.
Ri greater than  greater than h/e2≈26 Kxcexa9 (i=1, 2)xe2x80x83xe2x80x83(1) 
e2/Ct greater than  greater than kBT, Ct=C1+C2+Cgxe2x80x83xe2x80x83(2) 
Here, h is 6.63xc3x9710xe2x88x9234 J sec as Plank constant, e is 1.60xc3x9710xe2x88x9219 C as charge amount of electron, kB is 1.38xc3x9710xe2x88x9223 J/K as Boltsmann constant, and T is Kelvin temperature with a unit of K.
The mathematical formula (1) is a required condition of single electron tunneling to discern each event of tunneling each electron from another event. The formula (2) is a condition that the electron entered into the island blocks another electron with thermal fluctuations from entering into the island against Coulomb energy. These requirements mean that the impedance of the single electron device itself should be several hundred kxcexa9 as known in the formula (1), and that in order to operate the device at room temperature, the size of the island should be less than several decade nanometers and, as a result, the total capacitance Ct of the island 120 should be an order of aF (10xe2x88x9218 Farad), as known in the formula (2).
As described above, the essential features of the single electron device are the size of the island 120 and the good characteristic tunnel junctions 115 and 125. Here, the good characteristic tunnel junctions mean that the tunnel junctions should have the resistance Ri and capacitance Ci according to the formulae (1) and (2). At the present time, the fabrication methods of the single electron devices to satisfy these conditions are classified as two groups in terms of the used material: metals and semiconductors.
In the case of the metal material, Al or Nb is mainly used with double angle evaporation technique. At first, patterns are formed with a size less than several decade nm by electron beam lithography and metal is deposited to form the electron island and other electrodes ambient to the island. After that, the metal film is natural-oxidized to form a good oxide film on the surface. Subsequently, another layer of metal film is again deposited with slightly different angle to form the tunnel junction. This method is advantageous to fabricate a unit component. However, it is impossible to apply the method to the integration of single electron elements for practicing the single electron device, because of the complication of processes including three-stage levels and the limitation of double angle evaporation technique.
In the case of semiconductor, gates are fabricated on channel, using electron beam lithography (oxidation and etching in case of silicon) and the tunnel junction is inducible by the gate voltage. However, this also requires such several levels of fabrication processes that it has many difficulties in the integration of single electron elements.
As described above, the prior single electron device integration requires very difficult conditions in its fabrication. That is, it requires patterning technique of 10-nm level for operation at room temperature and tunnel junctions having a capacitance of about several aF and a resistance of about several decade kxcexa9. With the present technique, the fabrication of separate components can be proceeded to apply it to analog device such as sensor and detector, current standards and the like. However, the fabrication of the integrated digital circuit, which is more utilized and larger in demand, can not be proceeded with the prior material and processes.
In order to obtain the digital signal treatment of the single electron device and the utility as memory device, the integration of the single electron elements is essential. Thus, to achieve such requirements, it is very important to develop the fabrication processes of the single electron device to be easy and simple.
It is, therefore, an object of the present invention to provide a single electron device able to solve the above-described problems of the prior arts. The device has weak links with bottleneck figure in place of the tunnel junction of the prior device. The weak links are easily formed on the same substrate by simple processes and thus the integration of the single electron device can be easily achieved.
In accordance with an aspect of the present invention, there is provided a single electron device comprising: an insulating substrate; an ultra-thin metal film on the substrate; and a protecting insulating film on the metal film to protect the metal film, wherein the metal film comprises: a source region; an electron island coupled with the source region; a drain region coupled with the electron island; two weak links with bottleneck figure, through which the source and drain regions are coupled with the electron island, respectively, each of the weak links being inducible of the Coulomb blockade effect; and a gate electrode for providing a control voltage to control electric characteristics of the electron island.
In the single electron device of the present invention, the gate electrode may be capacitively coupled with the electron island laterally. In that case, the gate electrode and the electron island may be formed from the same metal film by etching it. However, the gate electrode may be formed on the insulating film capacitively to be coupled with the electron island vertically. The former is considered to be more preferable.
Otherwise, the gate electrode may be resistively coupled with the electron island. In that case, the gate electrode may be coupled with the electron island by means of a weak link, which is inducible of the single electron phenomena such as Coulomb blockade and Coulomb oscillations.
The weak links used in place of the tunnel junctions as well as for coupling the gate electrode with the island in case of the resistively coupled gate may be formed together with the source, the drain, the island and the gate electrode from the metal film by etching. At this time, the etching damage in the almost entire parts of weak links and the outmost parts of the other metal regions increases the resistance of the parts to at least 100 times. The increased resistance in the weak links can play a role of the tunnel junctions.
In accordance with another aspect of the present invention, there is provided a method for fabricating a single electron device, comprising the steps of: providing an insulating substrate; forming an ultra-thin metal film on the substrate; forming a protecting insulating film on the metal film to protect the metal film; and selectively etching the protecting insulating film and the metal film, in turn, to form a pattern of the metal film, wherein the pattern of the metal film includes a source region, an electron island, a drain region, two weak regions with bottleneck figure and a gate electrode, wherein the source and drain regions are coupled with the electron island through the two weak links, respectively, each of the weak links is inducible of the Coulomb blockade effect, and the gate electrode is for providing a control voltage to control electric characteristics of the electron island.